1. Field
The disclosed concept pertains generally to arc flash arresters and, more particularly, to arc flash arresters, such as, for example, shorting switches or other switching devices that arrest or quench an arc flash or arcing fault by closing open contacts.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems, which form part of an overall electric power system, include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within switchgear cabinets, such as between power buses, or between a power bus and a grounded metal component. Such arcing faults can produce high energy gases, which pose a threat to the structure and nearby personnel. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.
A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot be contained.
Recently, methods have been developed to minimize the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection, which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage.
In a medium voltage system, an internal arcing fault would occur somewhere inside of the switchgear enclosure, frequently, but certainly not limited to the point where the cables servicing the load are connected.
In a low voltage system, such as, for example, a motor control center, an internal arcing fault could occur within the load center panelboard when, for example, servicing live panelboards. A bare live copper bus could inadvertently be shorted. Another example for both low and medium voltage systems would be the shorting of the conductors by rodents, snakes, or other animals or objects.
In the low voltage system, the arcing fault could clear itself, by burning or ejecting the short, but it may take more than one-half cycle to do so, thereby causing significant damage and great risk of injury to workers even in one-half cycle of arcing.
A medium voltage system would behave similar to the low voltage system; however, the medium voltage system would be less likely to be self-extinguishing. The crowbarring of a shorting switch will extinguish the arc. Once the arc is out, and if the short has been burned away or removed, then system power can be restored.
It is known to employ a high-speed shorting switch to eliminate an arcing fault. Known arc elimination devices and systems produce a bolted fault across the power bus (e.g., phase-to-phase, such as two switches for three phases; phase-to-ground, such as three switches for three phases), in order to eliminate the arcing fault and prevent equipment damage and personnel injury due to arc blasts. It is also known to employ various types of crowbar switches for this purpose. The resulting short on the power bus causes an upstream circuit breaker to clear the bolted fault by removing power. See, for example, U.S. Pat. Nos. 7,145,757; 7,035,068; 6,839,209; 6,724,604; 6,693,438; 6,657,150; and 6,633,009. As a result, system power is lost due to the tripping of the upstream circuit breaker.
Known prior medium voltage shorting switches employ vacuum interrupters or vacuum envelopes having a partial vacuum therein.
Known prior low voltage shorting switches employ air at atmospheric pressure as an insulating medium.
There is room for improvement in arc flash arresters.